This invention relates to particle measurement techniques and, more particularly, to an apparatus and method for sizing and counting particles.
Airborne particulate matter, such as industrial aerosols and combustion-generated aerosols, generally span a range of particle diameters from about 0.001 to 10 micrometers (.mu.m). Because of the variable effect of particle size on visibility degradation, health, in-plant corrosion and so forth, it is advantageous to determine not only the total aerosol burden suspended in a gaseous medium (e.g., the number .mu.g/m.sup.3 of all suspended material), but the size distribution of that suspended material as well.
Currently, there is no adequate single technique available for continuously determining the size distribution of aerosols over the entire range of interest. Typically two instruments are utilized to span this range. For particles between about 10.sup.-3 and 1.0 .mu.m, particle size may be determined based on the electrical mobility of the particle, the instrument most often used for measurements of this type being the so-called "electrical aerosol analyzer". In this instrument, aerosol is first exposed to unipolar gaseous ions in a diffusion charger. The electrical mobility of the aerosol is then ascertained, from which the size distribution is then calculated. The upper limit of particle detection by this instrument is typically about 1.0 .mu.m.
A variety of optical detection techniques have been utilized in determining particles greater than about 0.1 .mu.m diameter. Both monochromatic and polychromatic light sources have been employed, as have a wide range of detection systems. Descriptions of optical detection methods are set forth, for example, in review articles that can be found in Optical Engineering, Vol. 19, No. 6 (1980).
Two separate instruments utilizing different principles of operation are accordingly needed in the prior art to determine the size distribution of suspended airborne particulate matter. It is among the objects of the present invention to develop a single particle counting apparatus and method which can measure particles over the entire size range of interest. A further object of the invention is to develop a technique to measure particle concentrations typical of atmospheric or industrial particle loadings. A still further object of the invention is to develop a technique which has application for in-situ measurement conditions.